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Variable stars in the VVV globular clusters. I. 2MASS-GC02 and Terzan10

The VISTA Variables in the Via Lactea (VVV) ESO Public Survey is opening a new window to study the inner Galactic globular clusters using their variable stars. These globular clusters have been neglected in the past due to the difficulties caused by the presence of an elevated extinction and high field stellar densities in their lines of sight. However, the discovery and study of any present variables in these clusters, especially RRLyrae stars, can help to greatly improve the accuracy of their physical parameters. It can also help to shed some light on the interrogations brought by the intriguing Oosterhoff dichotomy in the Galactic globular cluster system. In a series of papers we plan to explore the variable stars in the globular clusters falling inside the field of the VVV survey. In this first paper we search and study the variables present in two highly-reddened, moderately metal-poor, faint, inner Galactic globular clusters: 2MASS-GC02 and Terzan10. We report the discovery of sizable populations of RR Lyrae stars in both globular clusters. We use near-infrared period-luminosity relations to determine the color excess of each RR Lyrae star, from which we obtain both accurate distances to the globular clusters and the ratios of the selective to total extinction in their directions. We find the extinction towards both clusters to be elevated, non-standard, and highly differential. We also find both clusters to be closer to the Galactic center than previously thought, with Terzan10 being on the far side of the Galactic bulge. Finally, we discuss their Oosterhoff properties, and conclude that both clusters stand out from the dichotomy followed by most Galactic globular clusters.